Powder conveyance devices and methods are employed, for example, for conveying dosed quantities of a fine-grained powder for plasma coating processes. To prevent a standstill of the powder conveyance device, powder deposits and agglomerations are to be avoided.
The conveyance of fine-grained powders with a particle size of less than 150 μm with known conveyor devices is subject to considerable restrictions regarding the accuracy of the mass flow of the powder conveyed. There is no technically ideal solution for particles with grain sizes below 20 μm and feed rates of less of than 10 g/min. Below this grain size, the adhesion forces between the powder particles increase considerably. The surface of the particles in relation to the volume increases steeply. A cube with an edge length of 1 cm has a surface of 0.006 m2. The same volume stacked up with particles of five nanometers edge length has, however, a surface area of 2400 m2. The steep increase in the surface adhesion forces impairs the conveyance of such small particles. Agglomeration of the powder/gas mixture can be avoided by continuously coupling energy into the powder, for example, by sustaining high flow speeds, which involve a high gas or air consumption. Yet, high gas volume streams are disadvantageous for many kinds of subsequent working processes, such as, for example, plasma-coating processes or laser-coating processes. Furthermore, powder conveyance under high gas volume streams requires application of more energy. Also, small quantities of fine powders cannot be dispersed in gas.
German Patent Application No. 44 23 197 A1 discloses a powder pump for the spray-coating of articles in a rod-type elongated shape. On a front side, the powder pump carries a powder inlet opening via which the powder is drawn from an upwardly open powder container. The powder is subsequently conveyed via an inner tube of the powder pump to a consumer. The powder conveyance itself is driven by generating a vacuum within the powder pump. The vacuum is generated by an injector nozzle arranged in the vicinity of the powder inlet opening.
German Patent Application No. 10 2006 002 582 A1 discloses a powder conveyor with a fluidizing unit. The fluidizing unit is arranged at the end of a powder inlet tube of the powder conveyor and blows in the suction region of the powder inlet tube fluidizing air into the powder supply such that the powder is fluidized. The fluidized powder can be more readily sucked and conveyed. The fluidizing unit is arranged above and concentrically with the powder inlet opening. As a result, a screen-shaped region of fluidized powder is formed around the intake opening. The powder conveyance is performed by means of a compressed air injector, which is connected to the end of the powder intake tube opposite to the fluidizing unit. By construction of the powder conveyor, no fluidization occurs directly below the powder inlet opening. While the annular fluidization enhances the sucking of the powder, the powder conveyance lacks in constancy regarding the amount of powder sucked over time. Furthermore, powder agglomerates are merely deficiently dissolved, and for the most part, sucking directly through the powder suction opening. Powder agglomerates may cause issues and malfunctions, especially so in subsequent coating processes that aim for coatings of constant layer thickness.